1. Technical Field
This invention generally relates to the control of emissions from combustion processes such as coal-fired processes or other combustion processes that produce large quantities of solid fly ash particles. More particularly, the present invention relates to a system and method for segregating the separated fly ash particles based on particle size and collecting the segregated particles into individual storage containers. Specifically, the present invention relates to a separation system that allows ultra fine fly ash to be separately collected from a dry fly ash removal system by providing a dedicated removal system for removing fly ash particles only from the last separation device of the system.
2. Background Information
The combustion of coal and other similar fuels produces molten inorganic matter that is carried away in the exhaust gas stream as the fuel burns. The molten inorganic matter cools as the exhaust stream flows away from the combustion and coalesces into spherical or ellipsoidal ceramic particles in the general range of 0.01 to 500 microns in diameter. These combustion by-product particles are known in the art as fly ash.
Fly ash must be removed from combustion exhaust streams before the streams are exhausted to the atmosphere because of environmental concerns. Numerous methods and systems for removing fly ash are known in the art that effectively remove the fly ash from an exhaust stream. One problem common to all of these methods and systems is the cost of disposing of the collected fly ash. Although some of the fly ash may be sold for various commercial purposes such as for fillers, most fly ash must be landfilled at the expense of the company creating the fly ash. It is thus desired in the art to increase the quantity of commercially-valuable fly ash and consequently decrease the amount of fly ash that must be landfilled.
One known system that separates fly ash from a combustion exhaust stream is an electrostatic precipitator 12 depicted in FIGS. 1 and 2. Other systems 12 that will function with the concepts of the present invention are separation systems such as bag filters, cyclones, and others know in the art. Separation system 12 may be positioned as the final cleaning step for an exhaust stream from a coal-fired power production process. For instance, system 12 may be used in a power production process where coal is burned to create heat that produces steam to run generators. The burning coal creates an exhaust stream containing fly ash that must be substantially removed from the stream according to federal regulations. System 8 is a Fly Ash Removal System (FARS) which transports ash from the hoppers of the separation device 12. FARS 8 transports the fly ash pneumatically to a storage facility (i.e. silo, pond, etc). The conveying air can be either a pressure or vacuum system. In the example of system 8 depicted in FIG. 1, the ash enters system 8 directly from an inlet conduit 10 that receives ash from electrostatic precipitator 12 positioned above conduit 10 and receives its transport air from air intake 11.
Inlet 10 feeds a primary supply line 14 that is in communication with a plurality of separation units 16. In this embodiment of system 8, four separation units 16 are connected to primary supply line 14 in parallel such that each unit 16 receives substantially equal amounts of the fly ash-laden transport air. In other embodiments, a separate, individual precipitator may be used to deliver fly ash-laden transport air to each unit 16 without departing from the concepts of the present invention. Additional or fewer units 16 may be provided based on flow rate, need, and desired redundancies. Further systems 8 for one or more power producers may also utilize a silo 40 common to all systems 8.
Each separation unit 16 includes a feed line 18 that connects primary supply line 14 to a coarse separator 20. In this example, each coarse separator 20 includes a primary separator 22 connected to a secondary separator 24. In the preferred embodiment of the present invention, each separator 22 and 24 is cyclone. Feed line 18 is connected to the inlet of primary separator 22 such that the fly ash-laden transport air is drawn into the separation chamber of primary separator 22. The transport air exits primary separator 22 through an outlet 26 after at least a portion of the fly ash falls out of the transport air stream into a primary receiver 28. The separated fly ash exists through a first gate 30 that selectively opens and closes the outlet 32 to primary receiver 28. A storage hopper 34 is disposed below outlet 32 to collect the fly ash. The outlet 36 of storage hopper 34 is selectively opened and closed by a hopper gate 38. Storage hopper 34 is positioned above a collection silo or storage container 40 that gathers fly ash from each separation unit 16. A pressure equalization system 35 is provided to regulate the pressure in the system.
Outlet 26 of primary separator 22 is connected directly to the inlet 42 of secondary separator 24. Second cyclone 24 has an outlet 44 through which the transport air is drawn and a secondary receiver 46 where the fly ash removed by secondary separator 24 is temporarily collected. The outlet 48 of secondary receiver 46 is selectively opened and closed by a secondary gate 50 that selectively opens secondary receiver 46 to storage hopper 34.
Outlet 44 is connected to the inlet 52 of a tertiary separator 54. Each tertiary separator 54 is preferably a bagfilter in system 8. Other separators such as ceramic filters or other high efficiency separating devices known in the industry may also be used as the tertiary separator. Each bagfilter 54 is designed and configured to remove the smallest particles of fly ash from the transport air before it enters a vacuum pump 58. Bagfilters 54 thus function as a final cleaning step for the transport air. A bagfilter transfer conduit 56 connects outlet 44 of each coarse separator 20 to inlet 52 of bagfilter 54. The transport air flow is pulled through system 8 by vacuum pump 58.
As is known in the art, bagfilter 54 may often use a collection of fabric filters, similar to common household vacuum cleaners, but at a much larger scale, to entrap air-borne particulate matter onto a filter surface, allowing the largely particulate-free air to continue through the filter surface. During operation of bagfilter 54, particulate matter builds up on the surface of the filter. This buildup is commonly known as the bag's cake. Cakes are frequently allowed to build up to thicknesses of approximately 0.25 inch or somewhat more between intervals of cleaning. Bags in operational bagfilters are cleaned of cake buildup at periodic intervals that are determined by variables of operation and engineering design. The cleaning process often involves blowing air backwards through the bag filters, shaking the bags, or banging the tops of the bags, all of which cause a substantial portion of the filter cake to drop off the bags.
In system 8, the bags of each bagfilter 54 are cleaned by knocking the cakes off of the bags and dumping the cake material into an open hopper 60. First gate 61 selectively opens and closes into a transfer hopper 62. Transfer hopper 62 is selectively opened and closed by a second gate 64 that controls access to silo 40. The fly ash separated by bagfilter 54 is moved into hopper 62 and dumped into silo 40. A pressure equalization system 63 is provided to control the pressure in the system because the bag filter is under vacuum and the silo is not. As such, the fly ash collected in bagfilters 54 is mixed with the fly ash collected in coarse separator 20 and is commonly disposed. Silo 40 is emptied through a rotary conditioning drum 66 and into a vehicle 68 that transports the fly ash to another location.
It has been discovered as part of the present invention that the fly ash collected in bagfilters 54 is commercially valuable and that it is desirable to separately collect this fly ash. It has been found that the fly ash collected in bagfilters 54 comprises a plurality of particles with 90% of the particles having a diameter of less than 10 microns. It is known in the art that fly ash particles having diameters predominantly smaller than 25 microns are known as fine fly ash. U.S. Pat. No. 4,294,750 discloses the benefits of fine fly ash particles. The present invention refers to fly ash particles that are predominantly smaller than 10 microns in diameter as ultra fine fly ash. It is thus desired in the art to provide a system for collecting the fly ash from bagfilters 54 that does not combine the fly ash from bagfilters 54 with any fly ash from separators 22 and 24.